The present invention relates to a method and apparatus for distillation and in particular, to one utilizing a composite membrane through which evaporation and condensation takes place.
Conventional distillation requires heating the distilland to the boiling point of one or more of the constituents of the distilland and then recovering and condensing the resulting vapor to a liquid, the distillate.
Distillation has also been accomplished through the use of a thin, porous, lyophobic membrane separating the distilland, typically an aqueous solution such as salt water, from the distillate, such as pure water.
The terms lyophobic and lyophilic are employed here to describe the wettability of a liquid on a solid surface. Lyophobic describes a non-wetting behavior of a liquid such that the contact angle between a liquid and a solid is greater than 90.degree. so that the liquid tends to ball up and run off the surface easily. Lyophilic describes a wetting behavior of a liquid such that the contact angle between a liquid and a solid is less then 90.degree. and the liquid spreads over the solid easily.
The contact angle or wettability depends on the relative attraction between the solid and liquid and between the molecules of the liquid themselves. If the attraction of the liquid for the solid is more than half that for itself, the contact angle is less than 90.degree. and wetting is said to occur and the liquid is said to be lyophilic with respect to the solid. A liquid is lyophobic with respect to a solid when the attraction of the liquid for the solid is less than half that for itself thereby creating a contact angle greater than 90.degree..
When the liquid is water, the terms hydrophobic and hydrophilic are generally substituted for the more inclusive designations.
Due to the properties of the non-wetting lyophobic material at low pressures, liquid is prevented from entering within the pores of the lyophobic membrane. If the distilland has a higher equilibrium vapor pressure than the distillate, evaporation takes place at the hotter distilland side of the lyophobic pores and condensation occurs at the cooler, distillate liquid interface of the lyophobic membrane pores. Thus, in the case of the aqueous solution, in an effort to establish vapor pressure equilibrium, a net flux of water vapor thus passes through the pores of the hydrophobic membrane. One example of such thermal membrane distillation is described in U.S. Pat. No. 3,340,186.
This type of distillation process should be contrasted with reverse osmosis processes which also use porous membranes. Reverse osmosis does not involve an evaporation/condensation process. Thermal membrane distillation is an absolute filtration process where reverse osmosis is a relative filtration process.
Unfortunately, thermal distillation across a hydrophobic membrane has not been a practical approach for separating fresh water from salt water. This is due to a major problem called water-logging. Salt water migrates into the pores of the hydrophobic membrane and causes the fresh water and the salt water to make contact. In other words, the pores fill up with water thereby destroying the vapor barrier needed for the evaporation/condensation process.
The length of time it takes for water-logging to occur depends upon a number of variables. Typically, it only takes a few days for water-logging to occur and for the distillation process to stop completely. Although the membrane's vapor barrier can be re-established by draining the water from the cells and blowing the liquid out of the membrane pores with compressed air, having to do so every four to six days because of water-log greatly decreases the commercial applicability of thermal membrane distillation.